Electromechanics 2013 J Arthur Wagner PhD Prof Emeritus in EE wagneretalsbcglobalnet Fig 51 Magnetic field Amperes Law Amperes Law Right hand rule Magnetic intensity H is a ID: 273281
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Slide1
EE130Electromechanics2013
J. Arthur Wagner, Ph.D.
Prof. Emeritus in EE
wagneretal@sbcglobal.netSlide2
Fig. 5.1 Magnetic field: Ampere’s Law
Ampere’s Law
Right hand rule
Magnetic intensity H is a
line density
of magnetic field “caused” by current.Slide3
QuestionState Ampere’s Law
What “causes” magnetic intensity?Slide4
Magnetic Flux Density
Magnetic Flux is an
amount
of magnetic field,
webers
[
Wb
].Magnetic Flux density is an
area density of magnetic flux.Something like pressure, which is an area density of force.Slide5
QuestionWhat is the difference between magnetic flux and magnetic flux density?Slide6
Faraday’s Law
Flux is “caused” by voltage,
or voltage is “caused” by change of flux.Slide7
Fig. 5.8 Signs for Faraday’s Law
right hand rule for current and flux.
Load designation
,
i.e. the current
goes in
where the voltage
e is positive
.Slide8
Homework Chapter 5, Due Tuesday, Sep. 24. No Classes next Tuesday and Thursday. These days will be made up TBD.
Problems 5.1, 5.2, 5.3, 5.4, 5.9
Read Chapter 5 (even though we don’t show all of it in class)Slide9
Fig. 4.14 Switching voltage waveforms
Check
qa
,
vaN
, qb, vbN during Ts
/2.Discuss how vo
is formed from vaN and vbN. What is the ON time for vo
?Compare the first harmonic of vo with the first harmonic of vaN.Slide10
Voltage vaN
Pk-Pk
=
duty cycle =
period =
freq = Slide11
Voltages vaN and vbN
Pk-Pk
=
duty cycle =
period =
freq = Slide12
vaN, vbN, and vo = vaN
-
vbN
Pk-Pk
=
duty cycle =period =
freq = Slide13
vaN, vbN, and vo = vaN -
vbN
sign of
vo
bar =
direction of current io =Slide14
vo
Pulses are missing due to the display sample rate.
What is happening to the current
io
bar?Slide15
Permeability, relative permeability, and permeability of air
Flux density is linear with magnetic intensity in air, and
most other materials that we see around us.
Permeability is the proportionality constant.
The exception are materials with iron, Fe, cobalt, Co, nickel, Ni, and molybdenum (not so important).Slide16
Relative PermeabilityWhen people say, “permeability”, they usually mean “relative permeability”.Slide17
QuestionCalculate the permeability when the relative
permeability is 250.Slide18
Fig. 5.3 B-H characteristics
We have said, “Current causes magnetic intensity and voltage causes flux”. What ties these magnetic terms together? Answer: the material.
No magnetic material is linear, only
approximately. A B-H hysteresis loop is always present. Think of
Hm
as “exciting” the material below.Slide19
Fig. 5.4 Toroid with flux
How can we state the flux density in terms of Slide20
Fig. 5.5 Magnetic Structure with Air GapAll motors have an air gap. Why?
current
flux
magnetic material
air gap length